I answer that, In this matter we must make a distinction and see whether those who are to be baptized are children or adults. For if they be children, Baptism should not be deferred. First, because in them we do not look for better instruction or fuller conversion. Secondly, because of the danger of death, for no other remedy is available for them besides the sacrament of Baptism.
On the other hand, adults have a remedy in the mere desire for Baptism, as stated above (Article [2]). And therefore Baptism should not be conferred on adults as soon as they are converted, but it should be deferred until some fixed time. First, as a safeguard to the Church, lest she be deceived through baptizing those who come to her under false pretenses, according to 1 Jn. 4:1: "Believe not every spirit, but try the spirits, if they be of God." And those who approach Baptism are put to this test, when their faith and morals are subjected to proof for a space of time. Secondly, this is needful as being useful for those who are baptized; for they require a certain space of time in order to be fully instructed in the faith, and to be drilled in those things that pertain to the Christian mode of life. Thirdly, a certain reverence for the sacrament demands a delay whereby men are admitted to Baptism at the principal festivities, viz. of Easter and Pentecost, the result being that they receive the sacrament with greater devotion.
There are, however, two reasons for forgoing this delay. First, when those who are to be baptized appear to be perfectly instructed in the faith and ready for Baptism; thus, Philip baptized the Eunuch at once (Acts 8); and Peter, Cornelius and those who were with him (Acts 10). Secondly, by reason of sickness or some kind of danger of death. Wherefore Pope Leo says (Epist. xvi): "Those who are threatened by death, sickness, siege, persecution, or shipwreck, should be baptized at any time." Yet if a man is forestalled by death, so as to have no time to receive the sacrament, while he awaits the season appointed by the Church, he is saved, yet "so as by fire," as stated above (Article [2], ad 2). Nevertheless he sins if he defer being baptized beyond the time appointed by the Church, except this be for an unavoidable cause and with the permission of the authorities of the Church. But even this sin, with his other sins, can be washed away by his subsequent contrition, which takes the place of Baptism, as stated above (Question [66], Article [11]).